This invention relates to a multi-purpose lifting vehicle having a boom hoist device with a hydraulic elevation control, and more particularly, to a control system for said hydraulic elevation control.
Multi-purpose lifting vehicles are designed to accept various attachments to enable the vehicle to be utilized in lifting different types of material. For example, the vehicle may be used as a crane when outfitted with a tote boom attachment, or employed as a front end loader with a bucket attachment. In either case, the vehicle's boom hoist device is employed to raise or lower the boom or bucket respectively.
When the vehicle is utilized as a crane, the hydraulic fluid used as the actuating medium in the control system should be locked in the raising end of the hydraulic cylinder. The cylinder is attached to the boom hoist device for controlling the elevation thereof. The hydraulic fluid maintains pressure within the raising end of cylinder to prevent the load bearing hook attached to the boom hoist device from unintended downward movement.
When the vehicle is used as a front end loader, it is desirable to have the bucket and boom hoist device connected thereto "float", that is move downwardly or upwardly in accordance with changes of contour of the terrain over which the vehicle is advancing. Since the bucket, in the front end loader mode of operation, is generally in contact with the terrain, allowing the bucket to float prevents the bucket from losing contact with the ground when a depression is encountered, or raising the front axle of the vehicle if a rise in the terrain is encountered.
Accordingly, the need exists for a control for the hydraulic system which controls elevation of a boom hoist device regardless of the operating mode of the vehicle.